Biomass particle-radiation-interaction and the effect of shape and structure simplifications

Particle radiation significantly contributes to the overall heat transfer in pulverized solid fuel combustion. For a lower computational effort in numerical simulations, the particles are often assumed to be spherical without an internal structure. In particular, biomass particles deviate strongly from this shape assumption and consist of a structured porosity. To investigate the influence of these aspects, pulverized beech wood particles are modeled as realistic as possible, considering the asymmetric three-dimensional shape and tube-like pores. The numerically created particle is then used to calculate single-scattering properties (absorption-, scattering cross-section, and scattering phase function) by applying the discrete dipole approximation (DDA). Afterward, the shape and internal structure of the representative biomass particle are step-wise simplified, while comparing the impact on the radiation interaction with the detailed modeled particle.The calculations reveal that simplifications of the internal structure are acceptable as long as a similar shape is maintained. Here, an extruded ellipsis with no internal structure and an effective complex index of refraction indicates the best alternative and is recommended for future particle radiation simulations. While the deviations of the radiation cross-section between the extruded ellipsis and the realistic particle are below 10%, the deviations of the radiation cross-section between a sphere and the realistic particle are around 50%. The orientation averaged phase function of all particle models scatters more than 99.5% of the scattered radiation forward. Thus, a purely forward scattering model in combustion simulations is recommended.